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A quick and rather simple explanantion for the world appearing dark, while plants which require light as their basis of life still grow there in a similar way to ours is that the spectrum of light emitted by the sun of this world was shifted out of the visible range compared to your hero's world. Thus, there is a sun-like star above this world, but its light emitted appears to be 'dark' because most of it is not visible for the hero from another world used to a different light spectrum. Think of UV light and flourescence effects to get a basic picture of what this is about. All creatures in this world would also appear to have dark vision, as they evolved in the world with different light spectrum and thus can see normally in it.

An interesting perspective would be that some of the light spectrum visible to the hero might be invisible to the creatures of this world and he could mark locations, paths etc. with a flourescent substance which is visible in our visible spectrum, but not in this world's.

You say that the world is literally greyer and darker. I assume that includes the sky.

If we just filled the air with fine particles, light from the sun would get partially occluded, causing the entire world to be darker. It would also cause local light sources to appear weaker, meaning any lights the hero brings with them will also work less effectively.

This can be caused a number of non-magical ways. The particles could be ash from volcanoes or water vapour from a perpetual fog for example. Using ash or dust has the added bonus of making everything grimy as well.

Depending on how heavy your dust cloud is, it can even make it a little harder for your hero to breath, causing that extra level of discomfort in this horrible mirror world.

If your big bad is sitting ontop of a volcano, a very story-wise thing to do would be to stop the perpetual smoking of the volcano once he is defeated, allowing the world to slowly clear as the dust settles.

If you hold a piece of white paper at right-angles to a real mirror, you can see that the mirror image is slightly darker. If you measure the brightness of the Sun in a mirror it will be less bright than the real Sun. That is simply physics because (a) the light has to pass through glass and (b) the reflective layer will always absorb a certain amount of the light incident on its surface.

A mirror cannot itself produce light, all the light coming out of a mirror is a mere reflection of the light in the real world.

Obviously the mirror world is illuminated entirely by light that has come through glass from the real world. That alone makes it darker. What is worse however is the parts of the mirror world that don't have a mirror in them. They must receive their light purely from reflections from the mirror sky and from other objects. If they are far from a mirror they will get almost no light at all. They will receive some of course from shiny objects that reflect what little light they can but this will produce a diffuse gloomy effect.

Your requirement of trees and temperature being the same is a hard one under the constraint that everything has to seem darker and less colorful - both objective color and lightness depend on different (visible) wavelengths of light being present, and trees working like normal means that photosynthesis is up and running, so at least blue and red wavelengths need to be present in the usual quantity, making a monochromatic lighting (which would kill all colour, see Na-vapor-lamps) impossible. If the sun is its own self, and the temperature on earth is also fine, most of the longer (invisible) wavelength need to be coming though just fine, meaning a simple darkening of the atmosphere is out as well.

You'd need to have the hero's perception changed, either by changing the sensory input (magical lenses, magical/pharmacological constriction of the pupil) or the perception itself (spell that affects memory/senses).

I actually like the contracted pupil - pinhole pupils also being a nice cineastic effect. It could be effected via drugs that contract the circular muscle, or some sort of systemic drug (problem being that it's actually adrenaling that is widening the pupils, so a dark world should induce wide pupils in most people...) - With extremely small pupils, few light would reach the retina, making everything look dim, triggering the eye's dark adaption, making everything colorless. This effect would easily detectable in a mirror, so if more subterfuge is warranted, the Big Evil might magically place black particles in the pupil, dimming everything without being detectable without opthalmologic equipment.

Fairly simple answer but sounds kind of fun. Perhaps it is being absorbed. The mirror word is growing dark because it is fading back to where it came from. The "Big Bad Evil" has triggered the end of the mirror world, and now the prime world (real world the mirror reflects) is pulling the mirror world back into itself. The colors are fading and dimming because the light energy or whatnot is being pulled back into the real world.

This should meet your requirements. It take minimal magic, as in the bad guy could trigger the end and then just sit back and let it happen (at least until the good guys stop it and reverse the effect to save the day!). Temperatures can be similar or fluctuating depending on how the world is being 'absorbed' or whatever term you want to use. And the world falling apart can explain how it is a mirror world that does not reflect all changes that happen in the real world, (i.e. mirroring a big event takes lots of energy and the mirror world is losing energy so only small changes are reflected, or the opposite where only big energy changes are reflected).

This kind of story line also offers other things you can take advantage of such as if one world absorbs another, as they collapse do more gates between the worlds appear as one becomes unstable, or do elements from one bleed into the other. The list goes on.

Hope this helps.

The evil world is flat. The sun never rises or sets.

This world is a disc, turning on its axis. It stays with its edge facing its sun. On the ground, the sun is always moving along the horizon, never setting, never rising, never stopping. Sunlight is always redshifted and oblique. Shadows are long.

source

The Sun is life, the Sun is good. In some languages, like German, the Sun is even indicated with a feminine noun, to indicate its ability to bring life.

Your parallel world is evil. Evil is the opposite of good, or the lack of good.

So it is straightforward that in an evil world the sun light shines feebler or is obfuscated.

Make it multifactorial.

The very makeup of this mirror world is such that the atmosphere filters out more visible light, the matter itself absorbs more visible light, and some supernatural forces absorb some of the visible light (it is what powers them, and allows them to project their evil onto the world itself)

Perhaps it's a multiverseral effect of the dimensional travel or the spell to create the mirror world or gain access to it... either way, in another world, the color our perception of color is different enough that it takes on a different quality of brightness than our world. It can be an unwritten rule that utopia worlds are brighter compared to ours and distopias are darker. Not enough to be a hinderance to natives of the other worlds, but to a traveller, the difference can be stark or barely muted. To an expert traveler, the brightness or the colors will denote how much and in which way the verse has changed. Traveling to something very very close to our world will be a similar color... and we eventually learn this is the fabled "World without Shrimp" of lore.

This will also help you identify things that are not of this world as they would appear darker or lighter naturally. There could be a simple spell that all dimension travellers use to keep up appearances, so that you do not look extra-bright to natives and blend, but it needs to have a common failure point to give up the secret... perhaps those who correctly guess or are told will become immune to the deception.

It is caused as a side effect of the magical force.

The dark world has magic, which permeates everything. Under normal conditions this magic would be a neutral force. But that was before Big Bad Evil Guy rose to power, and imposed his will on magic, which in turn polluted the natural background magical field.

This causes everyone to tend to take on characteristics of Big Bad Evil Guy's personality to varying degrees, like putting a tinted lense over everything. Someone who would normally be sunny and super cheerful will instead be sullen. Someone who was prone to bad tempers and violence would become much more so, to the point where they might be even worse than Big Bad Evil Guy.

Going off Green's answer, another side effect of this is that perceptions of reality are also muted, so that while nothing in the world has really changed, the mental perceptions have, robbing hope, causing various degrees of depression.

There could be people who are naturally resistant, so that it affects them less.
And there could be places where the magical field is either weaker or stronger based on real world factors, so you could have a place that is especially dark, like bad gas building up in low lying swampy areas *.

Edit: This leads to a couple interesting possibilities; If the magical field is tainted, then it could lead to anyone using magic to become more and more evil, as concentrating the magic for use also causes the user to take on more of Big Bad Evil Guy's traits.

And if Big Bad Evil Guy is defeated, then it is possible that the magical field could be cleansed, returning light to the world, which is not something that can happen if it is a world with different physics or solar spectrum.

And finally, if Big Bad Evil Guy can impose his will on magic, polluting it, then it may also be possible that another powerful user could do the same, and push back on the darkness, creating an island of light where the darkness has no effect.

* does not have to be affected by altitude.

This is probably more magical than what you are looking for but given the constraints it's hard to come up with a good answer:

You say this world is permeated by evil. What is evil? It's a hive entity, the individual components of which appear as superfine quasi-transparent particles permeating the environment. While they do not interfere with photons that pass through the particles they do take variable amounts of energy from the photons. This runs the life processes of the evil and in the end shows up as heat which is released into the environment. Thus the thermal balance is not altered. Photosynthesis at ground level is slowed but the evil is mostly near the ground, treetops stick up high enough that they are pretty much unaffected.

Note that when you take energy from a photon (normally only possible by redshifting--intense gravity or extreme speed) you lower it's frequency and since color is based on frequency you change it's color. Since the effects are random it doesn't just shift the colors down the rainbow but smears them across the spectrum--and what's a mix of all colors but white? The smearing isn't perfect so some color remains but it's washed out.

Note also that this means that the more evil that's about the darker and less colorful the world will be--this could be useful to identify centers of evil.

The first thing that comes to my mind is the plot of MediEvil. The game begins when the Big Bad (Zarok) casts a spell plunging the country (make it the world) into darkness, and turning everyone into zombies. (As a side effect, he resurrects the hero who had been killed before).

In the mirror universe, the Big Bad has won against another hero. Casting such a spell must have been incredibly difficult and needed a lot of preparation, magic energy, combined relics, etc. In this mirror universe, the hero died and failed to stop the Big Bad, so he carried out his plan to turn the world into darkness and everyone into corrupted version of themselves. He would profit from this operation because with this spell, he can rule his world, or get vengeance over some king, or access incredible power through dark magic.

Why is only the mirror dimension plunged into darkness? Because the Big Bad has somehow a knowledge of there being two worlds and has or has had access to the mirror world. Using this, he has killed/merged with his mirror self and gained his power (that would also explain that there is no version of him in the "light" world). At least, he took or did something from the light world that people won't notice but would enable him to carry out his plan on the mirror world. Or maybe the two versions of the Big Bad agreed to unite to take over one of the worlds, being unstoppable with their combined powers. Eventually one of them was killed by the other over ruling disputes.

But basically, an incredibly powerful spell, that needs magic and power from both worlds to be cast, explaining why it's only present in one world.

Cons :

• It's entirely magic

Pros :

• The Bad Guy made it happen


• It can be reverted, by killing the Big Bad, destroying the crystal, ...

Mirror World Is Less Than the Real World

The laws of physics are very simply but profoundly different in mirror world -- 1 joule there is a tenth of a joule in the real world. Everyone from the mirror world sees everything as normal, but for someone from the real world, everything there emits much less light, is much less heavy and more delicate. They would seem preternaturally heavy, strong, and tough.

Now, I'm guessing that you don't want your hero to be practically unbeatable by the Mirror World natives. Presumably there's some sort of effect that reduces him to mirror person levels, although not completely (hence the place still looks dark). This could come from several sources:

• The portal is under the control of the BBEG, and anyone who doesn't work for him who goes through is automatically reduced


• The Mirror World has a slow corrupting effect which eventually turns you into a mirror person. Once you become a mirror person, you can't go back, because the light and heat of the real world would be so intense to you that it would destroy you. The only way the protagonist and his allies know to stop this from happening is a spell that also has the reducing effect


• The protagonist can't just run around in the Mirror World -- the intense energy of his body would set everything on fire! So he has to have the reducing effect applied to him in order to protect the people there

The Big Bad Guy spreads a virus/bacteria over the entire planet that influences your eyes/visual nerve/brain.

When you visit the planet, and pass through the portal, the virus immediately infects you, attacking your eyes, killing or disabling/temporarily poisoning most of the cells which are responsible for color vision, leaving mainly the ones for night vision(
Rod cells) intact.

Alternatively, the virus could attack your visual nerve or your brain, creating the same effect.

You quickly stop to perceive colors and see everything less bright and more blurry, recognizing the world far different than it really is.

This solution has the advantage that the world can be almost similar, but still seems greyish and dark to your character, so reality and it's perception differ by far.

Of course, if you defeat the Big Bad Guy, you can obtain the antidote/some medicaments against your virus.

Or, if you simply return to our world, the virus/bacteria is going to be killed by something very usual in our world. Like UV-Rays. Or some chemical substances which are in the air.

Or it depends on some gases which are in the air on the dark side.

As there are great answers regarding the dim light, this is only about people are meaner.

tl;dr: Cultures are self-enforcing and self-sustaining: Find an initial trigger that caused mistrust in society and you'll generate potentially very mean and egoistic people.

Behavior of individuals in society is a function of the culture of that society, and culture is the summary of all behaviors. Sociological and economic research has shown (in actual lab experiments, if I find them) that in standard trading games people behave nicely if everyone else does, too.

The summary of the experiment is roughly this: 10 (or so) players get randomly coupled up and will play a Game where you can choose either a "coop" or "egoistic" action. If both play coop, both get high outcomes. If one plays coop and one egoistic, the coop player gets nothing and the egoistic player gets more than in the previous case. If both play egoistic, both get very very little.

Notice that this game represents some important feature of culture: If everyone plays nice, the total outcome will be the highest. Mean people/free riders will benefit (earn more from free riding), but only if the remainder of society is nice. If everyone is mean, nobody profits. But also: If everyone is mean, it is the optimal strategy to be mean.

In the lab experiments, you get repeatedly matched with new people. You don't know how they played individually in the last rounds, but you know whether the total group of people was doing well or not. What happened there was that some groups by change ("inexplicably") turned into coop groups where everyone played nice, and some deteriorated. One of the determinants was group size: The larger the group, the more likely it deteriorates (similar as how people in smaller communities are more trusting than in large cities). Moreover: Move some from a "coop group" to a "egoistic group", and they will slowly learn and adjust.

So cultures are self-sustaining: Find an initial cause that made the culture to be bad and egoistic, and you'll find a self-enforcing mechanism. An initial group of mean people, perhaps followers of Evil Guy? At his arrival, Evil Guy Ponzi schemed the heck out of everyone and they are all now sad and lost and trust no-one? Perhaps in your world people look all very similar (dim light), so it's difficult to build up trust?

All of these mechanisms would be removed with the dimming: If Evil Guy is defeated, public information that his followers have left could turn society, if the cause was dim light so could the change of atmosphere.

A couple super volcanoes could do this. Spread enough ash into the air and you effectively blot out the sun. I think a Van Helsing show did this sort of thing for vampires to be able to come out. In that case it was based off a super volcano in NW America. However to make it magical and caused by the super bad he could have cast something to make this and maybe one other one on the other side of the world erupt, and continue to erupt and spew ash.

The psychological effects of being in darkness, in this case from ash cloud cover, are pretty well documented and could affect the populace. In addition you would be adding the fact that you could never really get anything clean, and I'm sure it would have health implications which would get worse as you get closer. Again leaning on the supernatural aspect you could say that part of the Big Bad's malice was being transferred in the ash and causing everything to be infected by it.

Like other said a volcano would darken the atmosphere.

The enemy of my enemy is my friend so maybe our intrepid hero gathers together a team/gang/club/"do gooders are us" to help him out. No matter how evil you are you will still hate Lex even more.

However, our lex luthor has a plan in mind, for world domination. He can't total wreck his world because he lives on it. He does have bunkers of course as a last resort.

He turn the world dark to black mail the residents of his world for full control. So Lex brings in trucks of special compound X, and triggers super volcano. Compound X is a nano cloud that feeds on the ash, but also alters it so only the shades of red and blue for growing can pass through, and infrared for heat.

Your world won't be pitch black, but with only red and blue passing through,it definitely going to be dark and eerie.

So the stage is set, and the whole world looks like below with infrared for heat and maybe some other color/spectrum. Lex is blackmailing the world, and its nearly hero time.

Now Lex is knows he is in for a long siege, and residents are not going to give in that easily. There is still backing stabbing on all sides. He will be attacked on all sides, but also has a plan for undoing it. A nearby river is prepared and safe guarded to dump on the volcano to extinguish it. Also the nano cloud, could be used to clean the sky in a few months. However, Lex will never do this for ages to ensure his dominance.

Enter our intrepid hero. Shooter guy. haha

Some of the Lex's forces he can sneak by, some he's going to have fight. After a long trek to the Evil lair the hero and his intrepid band of followers (if needed) comes to the control room. Lex, of course is waiting inside, and every one else fights his minions.

Lex, leads our hero along narrow passages,needless sharp cliffs, and other traps where he has prepared for the final confrontation. Either that, or some stupid complex torture chamber. Lets MacGyver that. Lex and hero guy end up using swords because Lex wants to kill him slowly. Through some surprise or whatever the hero ends up winning.(he stumbles, his Mom's name is Martha, hero talks him down, or any other movie trope.)

If desired you can force intrepid hero to hack Lex's computer, and any other shenanigans you wish

Alternative he can just push the "big red button".

The flood gates open, extinguishing the volcano. The nano clouds are re programmed,and start purifying the air and allowing light back into the world.

2nd alternative:

The hero has to rally a bunch of people who really hate Lex to divert a river, or cause an avalanche of snow, or some other way of putting it out.

Additionally, since the nano clouds are swarm controlled you can force the hero into all sorts of different hacking,bribing situations to your hearts content to break the encryption, and steal control of the swarm and purify the air.

Maybe the whole sky is one big nano cloud, and there's no volcano at all.

Maybe Lex disguised the nano cloud as pollution from Lex Corp's factories until it was too dense for any one to get rid of.

Space balls the password is "12345" no "Lex12345" hahahaha, maybe not "!Evil!Bad!Guy!12345:(muhahahaha" more secure

I have read darkness increases crime as its easier for criminals to operate unseen.

In the mirror world, an environmental or genetic factor damages the third type of photoreceptor present in people's eyes, which are little known/understood even today in this world.

New research shows that in addition to rods and cones, the eye has a third type of photoreceptor which is connected to mood. When these detect less light, e.g. because it's winter and days are shorter, the result can be a more dark mood.

If the denizens of the mirror world all took a journey to that world similar to your hero's, something about that journey could have damaged these photoreceptors. If they started out there, something about the common environment in the mirror world could be what causes the damage.

For example, a common environmental cause might be a particularly dazzling flash of light, or longer-term exposure to a particular wavelength of light we don't consciously register. That light could have come in a flash, like from the detonation of a nuclear or electromagnetic pulse weapon (or a flash on the journey to this world). It could have come from a solar storm that hit the planet. It could have come from a popular technology, like CRT or plasma or OLED televisions or early-generation smartphone screens, etc.

It's not at all hard to imagine that people adopt technology when the effects of that technology are incompletely known and unknowingly damage something they don't understand about how their bodies work, with delayed follow-on effects that are hard to definitively connect back to the actual source. Depending on how bad/smart the Big Bad Evil Guy is, he could be the one who developed that technology or promoted its adoption, fully knowing about the effect. To revert this at the end of the story, you could try an "enlightened population" route where people know about the effects and stop using the tech (e.g. Thalidomide) or have a newer better tech come out that by coincidence doesn't produce the ill effect (e.g. cathode ray tube displays, with their high pitched buzz), and permit natural healing. The enlightenment option could come as a result of defeating the Big Bad Evil Guy and uncovering his documentation about the effect and plans for maximizing it. Or, have the Big Bad Evil Guy defeated as an almost coincidental side effect of the new tech disruption. Because he's pushing his tech for other purposes, he'll refuse to adapt and double down on a failing investment, pushing himself into bankruptcy and damaging/destroying at least some of his own capacities for other evils.

The issue could also come from a virus or illness that is highly contagious (e.g. pink-eye) but otherwise relatively asymptomatic (or leaves sufferers with a very low level of chronic pain, like a dull headache), and because it's extremely common it's considered normal, so the negative consequences are underappreciated and there's very little research done into causes or fixes, as with many other common issues. To revert this at the end of the story, develop a cure.

If you want the Big Bad Evil Guy to have agency, CRISPR and similar gene editing techniques could have been used to release a virus or bacteria with the explicit intent of spreading dark moods and making the world more evil. He might have taken a lesson/template from Toxoplasma gondii, which messes with host animals' brains to promote self-destructive behavior that can help get the parasite into its preferred host (cats).

It could be a genetic change, or an epigenetic change, passed on from one generation to another. That one's harder to overcome at the end of the story, but some tech could be developed to help people see better, just like we can currently develop tech to let people see in normally invisible parts of the electromagnetic spectrum.

Whatever the issue is could affect rods and cones too, to make things more gray and less colorful, but that's not strictly necessary for the main effect.

To the extent that it's understood, you could even put this story in the present world. Maybe the main difference between our current society and the "Garden of Eden" was a genetic change in our visual system that prevents us from seeing the force of love/God or the existence of abundance, permitting scarcity narratives that divide and cause people to do all sorts of nasty things to each other.

Any sufficiently advanced technology is indistinguishable from magic, and advanced technology is (often) based on advanced understanding. There's a lot we don't understand (some of which, we are just starting to understand) about the forces that govern our universe and how we as humans interact with them. Touching on some of these themes could make the story more interesting, depending on the overall goals and ideas you have for your story. Good luck!

How about pollution/smog? The meaner people might not have cared about air pollution laws, like in some asian cities:

Or Evil Guys industrial empire produces it. Or there could be natural causes, or natural conditions that make smog much more likely to form.